SEXUAL DIFFERENCES IN RESPONSES TO SIMULATED HERBIVORY IN URTICA-DIOICA

We studied the responses of the stinging nettle, Urtica dioica, to simulated herbivory in terms of growth, branching, reproduction, biomass allocation and stinging trichome density. We focused on intersexual differences in the responses. The simulated herb-ivory treatment consisted of leaf blade clippings and removal of the plant apex. Extensive leaf clipping (50% of the leaf blades removed) carried out before anthesis had the most negative effect on reproduction in both sexes. In the other simulated herbivory treatments, male and female plants either compensated or overcompensated slightly for herbivory in terms of reproduction. The branching responses differed between the sexes. In females, both leaf clipping and apex damage increased branching. In males, contrary to females, apex damage decreased branching. Thus the compensation for losses due to simulated herbivory was caused by significant increases in the inflorescence dry mass of the branches, not of increased branching. Stinging trichome density on new leaves of females was higher than that on males, which could be explained by the increased demand for defence in females due to their higher and longer allocation to reproduction. In females, biomass allocation to reproduction and the stinging trichome density on the upper leaf surface correlated negatively. This indicates a trade-off between reproduction and structural defence in the female nettles.